This disclosure concerns an invention relating generally to methods and apparata for reducing emissions from internal combustion engines, and more specifically to methods and apparata for reducing particulate and NOx emissions from diesel engines.
Common pollutants arising from the use of internal combustion engines are nitrogen oxides (commonly denoted NOx and particulates (also known simply asxe2x80x9csootxe2x80x9d). NOx is generally associated with high-temperature engine conditions, and may be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas). This reduces the oxygen in the flame front and obtains a reduction in maximum combustion temperature, thereby deterring NOx formation. Particulates include a variety of matter such as elemental carbon, heavy hydrocarbons, hydrated sulfuric acid, and other large molecules, and are generally associated with non-optimal combustion. Particulates can be reduced by increasing combustion and/or exhaust temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NOx tend to increase particulate emissions, and measures which reduce particulates tend to increase NOx emissions, resulting in what is often termed the xe2x80x9csoot-NOx tradeoffxe2x80x9d.
At the time of this writing, the diesel engine industry is facing stringent emissions legislation in the United States, and is struggling to find methods to meet government-imposed NOx and soot targets for the years 2002-2004 and even more strict standards for 2008. One measure under consideration is use of exhaust after-treatment (e.g., particulate traps) for soot emissions control in both heavy-duty truck and automotive diesel engines. However, in order to meet mandated durability standards (e.g., 50,000 to 100,000 miles), the soot trapped must be periodically re-burned. This requires considerable expense and complexity, since typically additional fuel must be mixed and ignited in the exhaust stream in order to burn off the accumulated particulate deposits.
Apart from studies directed to after-treatment, there has also been intense interest in the more fundamental issue of how to reduce NOx and particulates generation from the combustion process. Studies in this area relate to shaping combustion chambers, timing the fuel injection, modifying the mode of injection (e.g, modifying the injection spray pattern), or tailoring the injection rate during injection so as to meet desired emissions standards. As an example of the use of tailored injection rate, U.S. Pat. No. 5,345,916 to Amman, entitled xe2x80x9cControlled Fuel Injection Rate for Optimizing Diesel Engine Operation,xe2x80x9d discusses how injection curves (i.e., variation in injection rates over time) can be modified depending on engine speed/load to optimize engine performance. Examples of injection curves wherein the injection rate decreases over time are illustrated (FIGS. 5B and 5D of Amman). Others have taken an opposite approach and proposed injection curves with increasing injection rates. Still others have proposed injection curves which are considerably more complex than simple ascending or descending curves, and which may increase during some period of charge injection and decrease over others; see, e.g., U.S. Pat. No. 5,425,341 to Connolly et al., wherein the shape of the injection curve varies in complexity depending on speed/load conditions.
Multiple injection, also called split injection, pilot injection, and post injection, has also been a proposed method for NOx and particulate emissions reduction in diesel engines (see, e.g., Tow, T., Pierpont, A. and Reitz, R. D. xe2x80x9cReducing Particulates and NOx Emissions by Using Multiple Injections in a Heavy Duty 0.1. Diesel Engine,xe2x80x9d SAE Paper 940897, SAE Transactions, Vol. 103, Section 3, Journal of Engines, pp. 1403-1417, 1994). A multiple injection engine varies from the standard xe2x80x9csingle injectionxe2x80x9d engine in that the injection of a single fuel charge during the combustion cycle is replaced by injection of several fuel charges spaced over time, with less fuel being used per injection so that the total amount of fuel finally injected per cycle is comparable to that used in single injection. By spacing fuel injection into several discrete spaced charges, burning and combustion temperature are more evenly maintained and combustion temperature is lower, which helps decrease emissions. While multiple injection is not in common use at the time of this writing, engines using the multiple injection concept are now in production or under development in Europe, Japan and the United States.
While multiple injection will assist the diesel engine industry in meeting emissions goals, it unfortunately does not appear to be a complete solution: it does not by itself decrease emissions to the minimum levels desired. There is thus a significant need for methods and apparata which assist in diesel engine emissions reduction.
The invention involves methods and apparata which are intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the methods and apparata. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
The invention relates to methods of, and apparata for, diesel engine fuel injection. A first fuel charge is injected into a diesel engine combustion chamber during a combustion cycle. Afterward, one or more subsequent fuel charges are injected at a higher injection pressure (and thus a higher injection rate) than the first fuel charge. Where more than one fuel charge is injected after the first fuel charge, each will have a higher injection pressure (and thus a higher injection rate) than the prior fuel charge so that the second fuel charge has a higher injection pressure/rate than the first charge, the third fuel charge has a higher injection pressure/rate than the second charge, and so forth. It is believed that this methodology will result in reduced soot and NOx emissions over single charge injection schemes using roughly analogous charge timing and volume (i.e., in injection schemes having a single charge injected over roughly the same timespan as in the multiple injection scheme of the invention, and with the same fuel charge volume). It is also believed that the invention will result in decreased soot emissions over those obtained in comparable multiple charge injection schemes using constant injection pressures between charges. As will be discussed elsewhere in this document, a multiple injection scheme with increasing injection pressures in successive fuel pulses is believed to provide increased combustion chamber mixing and soot oxidation rates. Particulate control is advantageously achieved within the combustion chamber using a straightforward modification of existing injection system hardware, without the need for cumbersome and expensive particulate traps. Additionally, Exhaust Gas Recirculation (EGR) can be used with the invention to provide for further NOx reductions. Thus, the invention allows simultaneous and significant reduction of both NOx and particulates emissions, which are (at the time of this writing) the key emissions of concern for diesel engine environmental standards. The multiple injection scheme may also result in better fuel economy than single injection schemes since the first fuel pulse in a series can be started slightly earlier than where only a single pulse is used, so that the greatest heat from the fuel pulse is released when the piston is optimally timed near top dead center.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.